Free L-2-pyrrolidone-5-carboxylic acid (PCA, pyroglutamic acid) was found to be a major water-soluble nitrogen compound in epidermis of several mammalian species. Its highest concentration was 186 micro moles/gm wet wt. for guinea pig epidermis and the lowest level was 20 micro moles in mouse epidermis. Even in this rodent the epidermal PCA concentration is similar to that of its more abundant free amino acids, such as alanine. The relatively high epidermal concentration is in contrast to that of guinea pig visceral organs (liver, spleen, pancreas, kidney) with PCA levels of about 2 micro moles/gm wet wt. or less. Experiments are planned to elucidate the origin, metabolism, and possible physiological significance of the high level of PCA in mammalian epidermis. To date, we have ruled out possible formation of epidermal PCA by: - a) spontaneous decomposition of glutamate or glutamine, b) conversion of D-glutamate to D-PCA by D-glutamate cyclotransferase, or c) possible microbial synthesis of PCA on the skin surface. The information obtained should not only lead to a better understanding of how the epidermis utilizes amino acids, but it may give us a clue as to the basis for the high level of free amino acids in this tissue.